This invention relates to crystal growth and more particularly to a method of liquid phase epitaxial growth of thin uniform layers.
The great majority of the multiple GaAs and Ga.sub.1-x Al.sub.x As layers used to form double-heterostructure (DH) diode lasers are grown by liquid phase epitaxy (LPE). In particular, they are prepared by the technique of equilibrium cooling, in which a Ga-rich growth solution saturated with As is placed in contact with a substrate wafer for a specified time while the system is being cooled at a uniform rate. The development of this technique has made it possible to grow smooth layers of uniform thickness (well below 1.mu.m, for the GaAs active layer) with controlled impurity concentration but not with reproducible thickness.
Even smoother LPE layers can be obtained if the growth solution is supercooled by about 5.degree. C. before being placed in contact with the substrate, as reported in a paper by the inventor in the Journal of Crystal Growth 27 (1974) 49-61. There are two different cases of this type, depending on whether the system temperature is reduced during the time of substrate-solution contact (supercooling technique) or is kept constant (step-cooling technique). It was shown experimentally that the thickness of the layers grown by both techniques, as well as by the equilibrium-cooling technique, was determined under the usual growth conditions by the amount of As that diffuses out of the growth solution to the substrate-solution interface.
These prior art methods of LPE crystal growth use a liquidus which is in equilibrium with the substrate on which the thin layer was to be grown. The growth is in the region where the growth mechanism is a diffusion limited process. Therefore, the layer thickness depends strongly on the growth time and is in fact proportioned to the three halves power of time for equilibrium-growth and to the half power of time for step-cooling growth. Reproducible growth of an LPE layer under equilibrium condition requires tight control of the growth parameters such as growth time, the exact solution temperature (to obtain equilibrium) and furnace temperature profile.
The disadvantage of these prior-art methods of growth is that the layer thickness cannot be controlled reproducibly. This lack of control not only affects the device performance because of the variation in layer thickness and the yield of useful devices is markedly reduced. In addition, these prior-art growth processes usually take a much longer time to grow the desired layer than in the method of the invention.
If it is desired to grow a thin (in the order of one-tenth micron) layer by the prior art methods, the growth solution was usually prepared to be in the equilibrium condition (i.e., the amount of supercooling of the growth solution is approximately 0.degree. C.). The time required for the growth was long; and if the solution was not actually in equilibrium or if the growth time was not properly controlled, layers of different thickness would result.
It is therefore the primary object of this invention to grow thin epitaxial layers which are highly reproducible.
It is a further object of this invention to provide a method of reproducibly growing a thin LPE layer on a substrate without tight control of the growth parameters such as growth time, exact solution temperature and furnace profile which are critical to the prior-art methods.
It is a further object of this invention to provide a method for growing a thin active layer in double-heterojunction laser diodes which will give low threshold current density and extended life time of the diodes containing such thin active layer.
It is a further object of this invention to grow films which have good surface morphology in addition to being thin and of uniform thickness.
It is a feature of this invention that these thin layers have high growth ratio which contributes to the extended lifetime of double-heterojunction laser diodes made with these thin layers.